The present invention is concerned with a skid-controlled brake system comprising a pedal-operated, preferably auxiliary-force-supported, brake pressure generator including a master cylinder connected to wheel brakes through main brake lines; a hydraulic auxiliary pressure supply system and wheel sensors as well as electronic circuits for detecting the wheel rotation pattern and for generating electric brake pressure control signals to control electromagnetically operable pressure fluid inlet valves and outlet valves provided in the pressure fluid conduits for skid control.
In some brake systems of the afore-described type (see U.S. Pat. Nos. 4,415,210 and 4,416,491), the brake pressure generator is a master cylinder coupled to a hydraulic brake force booster. The auxiliary pressure supply system includes a hydraulic pump and a hydraulic accumulator which, when the brake is applied, provides with the aid of a control valve, an auxiliary pressure in proportion to the pedal force. The dynamic pressure, through the master cylinder, is transmitted to the static brake circuits in communication with the master cylinder. The wheel brakes of one axle, preferably of the rear axle, are in direct communication with the pressure chamber and receive through the control valve, the pressure proportional to the pedal force. Moreover, for slip control, inlet valves are provided both in the static circuits and in the dynamic circuit, which inlet valves, normally, are switched to the passage position and by way of which, in the event of imminent locking of a wheel, the pressure fluid flow to the wheel brake concerned is discontinued.
Outlet valves are provided through which, if need be, pressure fluid flowing to the wheel brake can be directed toward the pressure compensating tank. Upon commencement of the skid control, through a so-called main valve, the booster chamber in which the controlled pressure admitted from the auxiliary pressure supply system prevails, communicates with the static brake circuits of the master cylinder to enable the amount of pressure fluid discharged through the outlet valves to be re-introduced to the static circuits. Moreover, for safety reasons, the piston in the master cylinder (or pistons if it is a tandem master cylinder) are restored or locked with the aid of a positioning means. Considerable structure is involved with generating, storing and controlling the hydraulic auxiliary pressure, with the dynamic in-flow into the static circuits and with insuring the proper operation of the brake upon failure of individual circuits.
The control signals for the inlet and outlet valves in brake systems of this type are generated with the aid of electronic switch circuits. The inputs these circuits are in communication with wheel sensors, e.g. inductive transducers, so that the circuits can respond to any change in the wheel rotation pattern indicating a locking risk. The responses can operate to keep constant, lowering or increase the pressure on the corresponding wheel.